Unformatted text preview: of age, every- Where Our Sexes Come From 55 thing shuts down for a bit. This is called the “juvenile pause,” when
the hypothalamus, pituitary, and gonads put their feet up and take a
well-deserved break.2 The hormone storm abates, bones grow, muscles
strengthen, brains wire and rewire themselves. But the sexual cyclone
is still on course, and in most of us it will make landfall by about age
Shortly before the final stage of puberty, neurons within our hypothalamuses rise from their slumber and, while the rest of our bodies
sleep, again begin to pump GNRH hormone into our pituitaries. Our
pituitaries respond by releasing follicle-stimulating and luteinizing hormones. Then, every night during sleep, about every ninety minutes our
gonads get a jolt of hormones. Ovaries begin to release estrogen, testes
create testosterone and dihydrotestosterone, and the shapes of our bodies begin to change.
Swelling of one or both areolae (the circle around the nipples) and
the first appearance of pubic hair along the labia mark the first brushstrokes of puberty in girls. Six to twelve months later that swelling has
spread to both breasts and reaches beyond the areolae. At the same time,
the pubic hair spreads to the pubic mound. About a year later, breasts
have enlarged to nearly final size, and pubic hair has assumed the adult
triangle-shaped appearance. Internally, another map is unfolding. The
vagina, uterus, and ovaries all respond to this new spate of hormones
as well. All enlarge and reach their final shapes and sizes. And, most of
the time, ovulation and menstruation begin.
In infants with one X and one Y chromosome, usually the penis
that formed before birth enlarges during the first four years of infancy,
and then rests. At about age eleven, the testes begin to produce testosterone, the penis and scrotum enlarge, and pubic hair begins to appear.
Ejaculation becomes possible early in puberty though ejaculates may
contain relatively few sperm. At about the same time, the penis enlarges,
and pubic hair becomes more abundant. About twelve months later
the adult, triangle-shaped growth of pubic hair appears. And in the last
stages, pubic hair spreads to the thighs and abdomen, facial hair begins
to grow, and voices deepen.
Most of the time. 56 Between XX and XY And most of the time all of these things happen because of those
once-every-ninety-minute pulses of hormones put out by our hypothalamuses—small pistons compressing and relaxing, driven by their
own biorhythms, pushing us, while we sleep, into adulthood. In
response to these periodic squirts of GNRH, our pituitaries begin a
cycle of on-off production of luteinizing and follicle-stimulating hormones. These hormones, in turn, seek out our testes or our ovaries and
push them to secrete the all-important sex steroid hormones, estradiol
In men, at the same time, another form of the enzyme 5-alpha reductase type 1 appears. This enzyme, much like the one produced during
fetal life, converts testosterone into 5-alpha dihydrotestosterone, another
key needed to open the last gates on the way to male development.
Again, the entire orchestra must contribute. If even a single instrument fails to deliver its part on time and in proper syncopation, the whole
symphony may crash. Hormones, enzymes, genes, neurons, receptors,
and signal transduction machinery can shatter into a million shards, and
the images those shards reflect can be as varied as our imaginations.
And as complex as all that may seem, it’s only part of the story.
The adrenal glands are another set of critical organs that form with
the genital ridge as it develops in fetuses. The adrenal glands also play
critical roles in the process of human sexual development. In response
to hormones produced by the hypothalamus and the pituitary glands
during fetal development and during puberty, the adrenal glands typically make and secrete several additional hormones. Among these are
the adrenal androgens—dehydroepiandrosterone (DHEA), androstenedione, and pregnenolone. These compounds by themselves don’t do
very much. But in the blood, enzymes convert these precursors into
the active hormones testosterone, dihydrotestosterone, and progesterone—hormones critical to our initial and continued development as
The exact role of androgens in female development is cloudy, but
many aspects of normal female development—including nerve and
muscle cell development, as well as hormone production—do not occur
normally in women with adrenal androgen deficiencies.4 Where Our Sexes Come From 57 Boys begin to produce DHEA at about age nine or ten and androstenedione a year or two later. With boys the adrenal androgens induce
underarm hair and some pubic hair, but do not appear to be involved
with other aspects of puberty. From birth on, the adrenal androgens act
like another set of cogs in the machine that drives male sexual development. And, as we will see later, when these adrenal androgens become
estrogens, which they can and sometimes do, men begin to look more
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This document was uploaded on 02/04/2014.
- Spring '14